Internal combustion engines can generally not start independently but rather require a minimum rotational speed of, for example, 60 to 200 rpm to start. For this purpose, a starter motor which has been fed from a battery, and which can output a starting torque even in a stationary state of the vehicle, is normally used. The substantial torques which have to be overcome by the starting torque during a starting process are the breakaway, the frictional torque and the compression torque. At the start of the starting process, a crankshaft of the internal combustion engine is stationary. As a result of friction points which are present, for example the cylinder raceways and/or bearings of the crankshaft, static friction occurs. For the crankshaft to rotate at all, this static friction must firstly be overcome in a first phase. For this purpose, a corresponding torque, which is also referred to as a breakaway torque, must be applied to the crankshaft. The necessary breakaway torque assumes large values, in particular when it is very cold and in the case of a long inactive period of the vehicle. If the starting torque of the starter motor which is applied to the crankshaft is not larger than the breakaway torque, the crankshaft remains stationary and the internal combustion engine cannot be started.
If the breakaway torque has been overcome, sliding friction now occurs at the above-mentioned friction points instead of the static friction. Although the sliding friction is reduced compared to the static friction, it also causes braking of the rotational movement of the crankshaft. The sliding friction forces which act between the pistons and the cylinder raceways are converted into a sliding friction torque here via a crank mechanism, which sliding friction torque fluctuates in the course of a working cycle. Furthermore, there is an additional friction torque which is independent of the piston movement and is linked directly to the rotation of the crankshaft, this being due, for example, to friction in the crankshaft bearings. In this second phase of the starting process, a certain portion of the starting torque must therefore be made available for compensating the sliding friction. In general, it is found here that, in a way which is analogous to the static friction, particularly large values of the sliding friction torque occur when it is very cold and in the case of a long inactive period of the vehicle.
The intention is that the crankshaft will be accelerated as the starting torque becomes effective, and a rotational speed which is necessary to start the internal combustion engine will be achieved. In addition to the sliding friction, the compression of the air mass in the cylinders which takes place on a regular basis as the crankshaft rotates must also be taken into account here. The compression of the air mass acts on the crankshaft as what is referred to as a compression torque. Within one working cycle of the internal combustion engine, this compression torque changes sign repeatedly. In the case of a four-cylinder four-stroke engine, the working cycle is 720°, that is to say two full rotations, over which a total of eight changes of sign take place. The angles at which a change of sign takes place are referred to here as equilibrium positions. In this context, a distinction is made between stable and unstable equilibrium positions, also referred to as dead centers. If it is attempted, for example, to rotate the crankshaft from a stable equilibrium position toward relatively large angles, the compression results in a negative restoring torque. When there is a rotation toward relatively small angles, a positive restoring torque occurs. In the case of unstable equilibrium positions, a slight deflection of the crankshaft in conjunction with the compression torque ensures that the crankshaft rotates toward a stable equilibrium position.
If the crankshaft is located in a stable equilibrium position at the start of a starting process, the compression torque acts in a braking fashion up to the point when a first top dead center is passed, i.e. counter to the starting torque. The compression torque subsequently acts alternately in a driving fashion and a braking fashion. Driving means that the compression torque acts in the same direction as the starting torque. As a result, the sum of the starting torque which is effective and of the compression torque produces the torque which is effective for accelerating the crankshaft, while the sliding friction torque described above always acts in a braking fashion.
If the starting torque which is effective is too small, the rotational speed which is necessary to reach a necessary rotational speed of the internal combustion engine may accordingly not be reached or even the first top dead center may not be passed. The crankshaft therefore comes to a standstill again. Such a case occurs, in particular, when the starting torque is too small owing to incorrect configuration of the starter motor and/or the sliding friction torque is too large owing to excessive cold and/or an excessively long inactive period and/or due to other causes.
The challenges described above ensure that a large torque is necessary to start an internal combustion engine, in particular when it is very cold and/or in the case of a long inactive period. Approximately 200 Nm may be stipulated as an order of magnitude for a necessary starting torque for a four-cylinder diesel engine.
In order to generate a necessary starting torque, a starter motor or so-called starter is usually used in conventional vehicles with internal combustion engines. As a rule, the starter is embodied as an electric machine. However, this has some disadvantageous effects. If the necessary starting torque of an internal combustion engine rises, the costs, the weight and the installation space required by the electric machine also increase approximately linearly. A further disadvantageous property of the electric machines used is that they are generally configured for rotational speeds in the four-digit range. These rotational speeds are much higher than are necessary to start internal combustion engines. A rapidly rotating starter or a rapidly rotating electric machine therefore requires a transmission unit. The transmission unit transmits a high rotational speed of the electric machine into a low rotational speed which is adapted to the internal combustion engine. A transmission ratio of the order of magnitude of 1:50 can be stipulated. The armature therefore rotates 50 times more quickly than the crankshaft. However, the armature also outputs only a 50th of the torque which is effective at the crankshaft. The integration of the transmission unit has in turn a disadvantageous effect on costs, weight and installation space.
Since the internal combustion engine itself reaches high rotational speeds (of the order of magnitude of 4000 to 7000 rpm) during operation, the electric machine also must not be permanently connected to the crankshaft. Owing to the mechanical coupling via the transmission unit, the maximum permissible rotational speed for the armature would otherwise be exceeded, leading to damage to the electric machine. In order to prevent the maximum permissible rotational speed of the armature from being exceeded in the locomotion mode of the motor vehicle, a force flux between the electric machine and the internal combustion engine is produced only for the duration of the starting process. For this purpose, elements such as, for example, a freewheel or a screwdriver or the like are used. These ensure that a change of sign cannot take place in the torque which is transmitted between the electric machine and the internal combustion engine. If only one rotational direction of the internal combustion engine (usually right-handed) is considered, the starter can exclusively drive the crankshaft (that is to say cannot brake the internal combustion engine) and the internal combustion engine can be exclusively driven (that is to say cannot cause the starter to operate at overspeed).
In a motor vehicle with a hybrid drive, an electric machine which is provided to drive the vehicle can, if appropriate, completely assume the function of the starter. As has already been made clear above, for this it is particularly necessary that the electric machine can output a sufficiently high torque for the starting process. However, this leads to a configuration conflict, in particular for what are referred to as mild hybrid vehicles which are equipped with relatively weak electric machines and small energy stores. The relatively weak electric machines generate a maximum torque of the order of magnitude of 100 Nm here.
A known solution for mild hybrid vehicles is to install a conventional starter, that is to say a second electric machine, which is used, in particular, at extremely low temperatures and/or in the case of a long inactive period of the vehicle. Another approach to a solution is to use a relatively strong electric machine which can generate a larger maximum torque. Apart from the elimination of the starter or of the second electric machine, the achievable advantages here are, however, limited if the capacity of the electric energy stores is not simultaneously increased, which would result in what is referred to as a full hybrid drive.
The specified alternatives of “mild hybrid vehicle with conventional starter”, “mild hybrid vehicle with over-dimensioned electric machine” and “full hybrid vehicle” cannot be sensibly implemented in many hybrid vehicles for technical and/or economic reasons (costs, weight and/or installation space). In particular small and medium-sized vehicles are affected by this. Nevertheless, in the medium term the aim is to hybridize these vehicles too, or to at least consider hybridizing them.
The technical problem therefore arises of providing a method and a device for starting an internal combustion engine in which a maximum starting torque of a starter motor is reduced compared to conventional starters or electric motors, as a result of which costs, installation space and energy demand of the starter are reduced. In particular, this technical problem arises for what are referred to as mild hybrid vehicles in which the electric machine which is present for providing drive is to function at the same time as a starter.